Talk:Transactions

Discussion of the feasibility and desirability of implementing transactional concurrency in Eiffel.

Examples please

I think we need to see some examples (well, I do for one).

The main thing to remember is that the proposal states that all features calls are asynchronous.

Transactions are assured with transactional memory, probably implemented as software transactional memory.

Example of transaction:
feature
correct_removal(container: DISPENSER) is
 transaction -- This is a transaction so it is assured to be atomic and isolated
  if
   not(container.is_empty)
  then
   container.remove
  end
 end

This feature is assured to be correct because is is isolated and atomic with respect to the system. If the container has been modified between the call to is_empty and remove, the feature would be aborted and retried.

feature
incorrect_removal(container: DISPENSER) is
 do
  if
   not(container.is_empty)
  then
   container.remove
  end
 end

This is the standard example of concurrency issues with preconditions. The container can change between the call to is_empty and remove. Using transactions avoids this.

Example of concurrency:
feature
write_all_files is
 do
  write_file_1
  write_file_2
  write_file_3
  write_file_4
 end

Since all feature calls would be asynchronous, the four write_file features could be executed in parallel if the runtime decides to do so. This is not a transaction so isolation across these calls is not assured.